Method for press hardening of metals

ABSTRACT

The invention relates to a method for press hardening of a metal or an alloy wherein said metal is heated in a controlled furnace atmosphere which is essentially hydrogen-free and which comprises nitrogen and carbon monoxide.

The invention relates to a method for press hardening of a metal or analloy wherein said metal or alloy is heated in a furnace and the hotmetal or alloy is formed and cooled in a die.

The composition, function and control of the furnace atmosphere are ofcrucial importance for the result of all heat treatments.

In the following, the term annealing is used for heat treatment ofmetals or alloys which requires a controlled atmosphere, and where theaim is to produce certain microstructures and properties. Annealing isdone both in steel and nonferrous milling plants.

In the manufacturing industries annealing furnaces have a wide range ofsizes and designs: from a small box furnace in a tool room, to a bigcontinuous isothermal annealing furnace in the automotive transmissionworkshop, from the advanced vacuum furnace in the aerospace industry, toair filled furnaces for lower demand industries.

Hardening is a heat treatment procedure for steels with the aim toproduce a martensitic or bainitic microstructure. Hardening is normallydescribed separately from annealing but the furnace atmosphererequirements are similar for both processes. Hardening involves thesteps of heating up to a temperature above the temperature, where thesteel is austenitic, followed by a fast quench. The austenite willtransform upon quenching to the hard microstructure martensite orbainite if the quenching rate is fast enough.

The so-called press hardening method, also known as hot stamping or hotforming, is used to produce parts of extremely high strength. The metalor alloy is heated up to a temperature between 800 and 1000° C.,introduced into a die and then subjected to a hot forming process. Theterm “die” shall mean any kind of die, stamp or press which can be usedto change the form of a hot metal object. In the die the metal object isformed hot and then cooled and whereby hardened. The forming step andthe cooling and hardening step are carried out within the die. Soproduced steels show a very high tensile strength.

To avoid the negative effects of oxidation, carburizing, decarburizationor nitriding during annealing or hardening, the atmosphere must becontrolled with respect to the annealed or hardened alloy. The carbonpotential of a furnace atmosphere is equal to the carbon content thatpure iron would have in equilibrium with the atmosphere. The carbonactivity (aC) of a furnace atmosphere is the carbon content a metal oralloy would have compared to the reference, graphite, defined as beingequal to aC=1. Both the carbon activity and the carbon potential in heattreatment processes affect the final properties of the metal/alloy inmany ways.

In order to avoid the creation of metal oxides on the surface of theheat treated metal or alloy the heat treatment process is often carriedout in a reducing atmosphere. For example a heat treatment atmospherefor annealing of copper wire coils might contain nitrogen with 3%hydrogen.

Besides the described synthetic nitrogen-hydrogen atmosphere, today theatmosphere is often based on a combination of nitrogen and endogas. Ifthe endogas is made from natural gas it may contain up to 40 vol % ofhydrogen, some carbon monoxide (ca 20 vol %), carbon dioxide and water(ca 0.3-1 vol %) with the remainder being nitrogen.

In neutral annealing and neutral hardening, e.g. in press hardening orhot forming of steel parts, a reducing atmosphere has to be used inorder to avoid oxidation of the steel surface. A typical reducing agentis hydrogen. However, especially when a high-strength steel is subjectedto a hydrogen containing atmosphere hydrogen embrittlement may occur,that is the steel becomes brittle and might crack.

Thus it is an object of the invention to develop an improved method forpress hardening of metals or alloys.

This object is achieved by a method for press hardening of a metal or analloy wherein said metal or alloy is heated in a furnace and the hotmetal or alloy is formed and cooled in a die, which is characterized inthat nitrogen and carbon monoxide are introduced into said furnace tocreate a controlled furnace atmosphere and in that no hydrogen isintroduced into said furnace.

According to the invention the metal or alloy is heated in a furnace andthen the hot metal or alloy is directly transported into the die whereit is formed and cooled. In the furnace a controlled atmosphere is usedwhich is essentially free of hydrogen and which comprises nitrogen andcarbon monoxide. The concentration of carbon monoxide in nitrogen couldbe between 0.1 and 99 vol %. The proposed atmosphere has no or only lowdriving force for de-carburization. The invention provides a neutralatmosphere for heating metals for subsequent press hardening.

The heating process is carried out in a controlled furnace atmospherewhich is essentially hydrogen free. “Essentially hydrogen free” meansthat no hydrogen gas is introduced into the furnace. Depending on thecomposition of the feed gas to the furnace it is sometimes unavoidablethat small amounts of hydrogen are produced in the furnace by reactionof the components of the feed gas with each other. However, according tothe invention the feed gas to the furnace does not contain hydrogen gas.

The inventive furnace atmosphere comprises nitrogen and carbon monoxide.Both gases, nitrogen and carbon monoxide, are directly introduced intothe furnace rather than being produced in the furnace. That is, the feedgas to the furnace comprises nitrogen and carbon monoxide. By directlyfeeding nitrogen and carbon monoxide into the furnace the composition ofthe furnace atmosphere can be better controlled.

Especially in hardening processes the metal or alloy is rapidly cooled,especially by gas cooling, after the heat treatment process. The coolingis preferably achieved by quenching the metal parts by means of a coldprotective gas. In press hardening the metal or alloy is heated in thefurnace and then directly transported into the die for hot forming.During the hot forming process the metal object is cooled, preferablyrapidly cooled, in order to harden the metal object. The cooling rate ispreferably chosen to be more than 30° C./sec, more preferred more than40° C./sec. It has been found that in this way a hard martensiticmicrostructure is achieved.

The inventive furnace atmosphere is in thermodynamic equilibrium. Thus,it is possible to implement a process control using an external heatedoxygen probe or a gas analyser measuring carbon dioxide in combinationwith measurements of the carbon monoxide level and the processtemperature.

The invention is preferably used for press hardening of boron containingsteel. Further alloying elements such as chromium, manganese, silicon,nickel, molybdenum, cobalt or tungsten might be present in the steel.Boron containing steels or boron alloyed steels are very sensitive tohydrogen. According to the invention no hydrogen gas is fed into thefurnace. Thus the risk of hydrogen embrittlement is avoided or at leastminimized. The invention provides a solution for heating up boroncontaining steels before press forming without dissolving hydrogen inthe steel.

The invention provides an atmosphere for neutral annealing and neutralhardening. That means, there are essentially no net reactions betweenthe atmosphere and the metal. The inventive atmosphere is neutral withrespect to carburization, that is undesired de-carburization as well ascarburization are avoided. Metal oxides, in particular surface metaloxides, are reduced and oxidation is prevented.

In a preferred embodiment a carbon containing enrichment gas is added tothe furnace atmosphere. The term “enrichment gas” shall mean a gas whichallows to adjust the carbon potential or carbon activity to a pre-setvalue, Preferred enrichment gases are acetylene, propane and/or methane.

The inventive atmosphere may be advantageously produced by one of thefollowing methods:

-   -   Removal of hydrogen from endogas:    -   In order to create the inventive atmosphere hydrogen is removed        from the endogas. This is preferably achieved by using        adsorption techniques, in particular a PSA (pressure swing        adsorption) process.    -   Removal of hydrogen from syngas:    -   Syngas or synthesis gas, is the name given to gases of varying        composition that are generated in coal gasification and some        types of waste-to-energy gasification facilities. Syngas        consists primarily of carbon monoxide and hydrogen. By removing        the hydrogen from the syngas an inventive atmosphere is created        which has a high carbon monoxide concentration.    -   Removal of hydrogen from cracked methanol since cracked methanol        could be regarded as a syngas with the composition of 33% CO and        67% H₂.    -   Production of carbon monoxide with added air over a heated bed        of doped graphite:    -   Air or impure nitrogen with a quality containing residual oxygen        levels up to 3% is used and the contained oxygen is caused to        react to carbon monoxide inside the furnace over a graphite or        coal bed or in an external coal filled reactor.    -   Production of carbon monoxide by dissociating formic acid        injected into a heated reactor filled with sulphuric acid or        phosphoric acid. The formed carbon monoxide is then dried from        water and scrubbed to reach neutral pH-value.

Since the inventive atmosphere is hydrogen free the above mentionedproblems with hydrogen embrittlement are avoided. Thus the invention isparticularly useful in hardening processes where the parts are nottempered after quenching which could take place in cooled tools,submersion in water, oil polymer, molten salt or in gas. Examples ofsuch processes are hot forming of steel or press hardening of steel.Oxidation of the steel surface is prevented by the inventive addition ofcarbon monoxide to the atmosphere which has a reducing effect.

EXAMPLES

The invention provides a solution for a reducing controlled atmospherefor annealing or hardening coated steel, which in hydrogen containingatmosphere could dissolve hydrogen into the steel.

Another example of processes where the invention is used with advantageis in manufacturing of fasteners, e.g. screws, bolts, nails, nuts. Theseproducts are subjected to a high static load during the lifetime, acondition where atomic dissolved hydrogen could be a potential riskespecially when tempering of the martensite is done at low temperature.

Another example relates to bainite hardened steels which are quenched inmolten salt or hot gas. The inventive hydrogen free atmosphere couldpositively affect the austenitising process, e.g in the production ofbearing steels.

1. Method for press hardening of a metal or an alloy, wherein said metal or alloy is heated in a furnace and the heated metal or alloy is formed and cooled in a die, comprising introducing nitrogen and carbon monoxide into said furnace for controlling an atmosphere of said furnace without hydrogen being introduced into said furnace.
 2. Method according to claim 1, wherein said furnace atmosphere comprises a carbon containing enrichment gas.
 3. Method according to claim 2, wherein said enrichment gas is acetylene, propane and/or methane.
 4. Method according to claim 1, comprising removing hydrogen from syngas, from endogas or from cracked methanol for producing said nitrogen and carbon monoxide.
 5. Method according to any of claim 1, comprising reacting a nitrogen containing gas containing less than 10% oxygen over graphite for producing said nitrogen and carbon monoxide.
 6. Method according to claim 1, comprising injecting formic acid into a heated reactor filled with sulphuric acid or phosphoric acid and dissociating said formic acid for forming said carbon monoxide.
 7. Method according to claim 1, comprising measuring carbon monoxide and temperature of said furnace atmosphere by a heated external oxygen probe or a carbon dioxide gas analyser for controlling the heating process.
 8. Method according to claim 1, wherein said furnace atmosphere comprises between 1% and 5% CO, between 90% and 99% N₂ and between 0.05% and 1% hydrocarbon gas.
 9. Method according to claim 1, comprising subjecting said metal or alloy to a hardening process comprising heating said metal or alloy and quenching said metal or alloy.
 10. Method according to claim 1, further comprising manufacturing fasteners made of metal or alloy in said furnace atmosphere. 